This invention related to an engine control apparatus and, more particularly, to an engine control apparatus which can provide improved fuel economy and improved exhaust performance at high-speed and high load conditions.
For example, Japanese Patent Kokai No. 63-41634 discloses a fuel delivery control apparatus for controlling the amount of fuel metered to an internal combustion engine. The fuel delivery control apparatus employs a digital computer for calculating a desired value for fuel delivery requirement in the form of fuel-injection pulse-width and timing. A basic fuel-injection pulse-width value Tp is calculated by the digital computer central processing unit as Tp=K.times.Q/N where K is a constant, Q is the intake air flow and N is the engine speed. The calculated basic value Tp is then corrected for various engine operating parameters. The corrected fuel-injection pulse-width value Ti is given as EQU Ti=Tp.times.COEF.times.ALPHA+Ts
where ALPHA is a correction factor related to the oxygen content of the exhaust gases for providing a closed loop air/fuel ratio control, Ts is a correction factor related to the voltage of the car battery, and COEF is a correction factor given as EQU COEF=1+KTw+KMR+KAS+KAI+KFUEL+. . .
where KTw is a correction factor decreasing as the engine coolant temperature increases, KMR is a correction factor related to a desired air/fuel ratio, KAS is a correction factor for providing fuel enrichment control when the engine is cranking, KAI is a correction factor for providing fuel enrichment control when the engine is idling, and KFUEL is a correction factor for providing fuel enrichment control when the engine is accelerating. The calculated values for fuel-injection pulse width and fuel-injection timing are transferred to a fuel-injection-control logic circuit. The fuel-injection-control logic circuit then sets the fuel-injection timing and fuel-injection pulse-width according to the calculated values for them.
The air/fuel ratio is not required to satisfy the stoichiometric value over the entire engine operating range particularly for supercharged engines. It is desirable to suppress an excessive exhaust gas temperature increase at high-speed and high-load conditions by operating the engine at an air/fuel ratio richer than the stoichiometric value. It is also desirable to save fuel consumption by operating the engine at an air/fuel ratio leaner than the stoichiometric value. For example, Japanese Patent Kokai No. 60-19939 discloses a fuel delivery control apparatus for resuming a closed loop control to adjust the air/fuel ratio at the stoichiometric value after operating the engine at a lean air/fuel ratio for a predetermined period of time or when the catalytic converter temperature exceeds a predetermined value. With such a conventional fuel delivery control, however, the air/fuel ratio is retained on its rich side at high-speed and high-load conditions even though the exhaust temperature does not increase to a sufficient extent, for example, during transient conditions. This results in poor fuel economy and increased emission of CO and HC pollutants.
Japanese Patent Kokai No. 61-55340 discloses a fuel delivery control apparatus arranged to retain the air/fuel ratio at an economy value at high-speed and high-load conditions as long as the exhaust gas temperature is below a predetermined value. However, this fuel delivery control cannot retain the engine output torque at a target value.